I've noticed the same thing, when the head is on the eVod the wicking is perfect, no dry hits, plenty of vapour, etc. The same head on the protank gives me dry hits.
What I've noticed is that air can get trapped on the bottom end of the clearo, and the draw is way to airy. So I tried to either pull a small draw or suck air as much as possible, both options give me dry hits though the same heads work fine on the eVod, be it default silica or custom made cotton. Liquids tested were 80pg/20vg and 50/50, I'm definitely pin pointing this on the protank head. Such a shame, the liquids look so good in that tank...
Did anyone else stumble across issue? If yes, how did you fix it? I'm thinking it could be a design flaw, specifically the tight space where the wick ends have to fit into, combined with the possible air trapping in the lower part of the middle post (the chrome one that goes in between the bottom screw and the glass tank).
The weirdest thing is that even on a very fluffy and full cotton wick, the air draw stays the same. I'm suspecting that air might be mixing in from somewhere, though I can't say where from since the bottom is completely screwed on the tank and the airholes would be the only places where air can get in. What do you guys think?